Isolation levels and concurrency

Derby provides
four transaction isolation levels. Setting the transaction isolation level
for a connection allows a user to specify how severely the user's transaction
should be isolated from other transactions.

For example, it allows you to specify whether transaction A is allowed
to make changes to data that have been viewed by transaction B before transaction
B has committed.

A connection determines its own isolation level, so JDBC provides an application
with a way to specify a level of transaction isolation. It specifies four
levels of transaction isolation. The higher the transaction isolation, the
more care is taken to avoid conflicts; avoiding conflicts sometimes means
locking out transactions. Lower isolation levels thus allow greater concurrency.

Inserts, updates, and deletes always behave the same no matter what the
isolation level is. Only the behavior of select statements varies.

To set isolation levels you can use the JDBC Connection.setTransactionIsolation method
or the SQL SET ISOLATION statement.

If there is an active transaction, the network client driver always commits
the active transaction, whether you use the JDBC
Connection.setTransactionIsolation method or the SQL SET ISOLATION
statement. It does this even if the method call or statement does not actually
change the isolation level (that is, if it sets the isolation level to its
current value). The embedded driver also always commits the active transaction
if you use the SET ISOLATION statement. However, if you use the
Connection.setTransactionIsolation method, the embedded driver commits
the active transaction only if the call to Connection.setTransactionIsolation
actually changes the isolation level.

These levels allow you to avoid particular kinds of transaction anomalies,
which are described in Transaction Anomalies.

Table 2. Transaction Anomalies

Anomaly

Example

Dirty Reads

A dirty read happens when a transaction
reads data that is being modified by another transaction that has not yet
committed.

Transaction A begins.

UPDATE employee SET salary = 31650
WHERE empno = '000090'

Transaction B begins.

SELECT * FROM employee

(Transaction
B sees data updated by transaction A. Those updates have not yet been committed.)

Non-Repeatable Reads

Non-repeatable reads happen when
a query returns data that would be different if the query were repeated within
the same transaction. Non-repeatable reads can occur when other transactions
are modifying data that a transaction is reading.

Transaction A begins.

SELECT * FROM employee
WHERE empno = '000090'

Transaction B begins.

UPDATE employee SET salary = 30100
WHERE empno = '000090'

(Transaction B updates rows viewed by transaction
A before transaction A commits.) If Transaction A issues the same SELECT statement,
the results will be different.

Phantom Reads

Records that appear in a set being read
by another transaction. Phantom reads can occur when other transactions insert
rows that would satisfy the WHERE clause of another transaction's statement.

Transaction B inserts a row
that would satisfy the query in Transaction A if it were issued again.

The transaction isolation level is a way of specifying whether these transaction
anomalies are allowed. The transaction isolation level thus affects the quantity
of data locked by a particular transaction. In addition, a DBMS's locking
schema might also affect whether these anomalies are allowed. A DBMS can lock
either the entire table or only specific rows in order to prevent transaction
anomalies.

TRANSACTION_SERIALIZABLE means
that Derby treats the transactions
as if they occurred serially (one after the other) instead of concurrently. Derby issues locks to prevent
all the transaction anomalies listed in Transaction Anomalies from
occurring. The type of lock it issues is sometimes called a range lock.

TRANSACTION_REPEATABLE_READ

RS from
SQL.

TRANSACTION_REPEATABLE_READ means that Derby issues
locks to prevent only dirty reads and non-repeatable reads, but not phantoms.
It does not issue range locks for selects.

TRANSACTION_READ_COMMITTED

CS or CURSOR
STABILITY from SQL.

TRANSACTION_READ_COMMITTED means
that Derby issues locks
to prevent only dirty reads, not all the transaction anomalies listed in Transaction Anomalies.

TRANSACTION_READ_COMMITTED is
the default isolation level for transactions.

TRANSACTION_READ_UNCOMMITTED

UR, DIRTY
READ, or READ UNCOMMITTED from SQL.

For a
SELECT INTO, FETCH with a read-only cursor, full select used in an INSERT,
full select/subquery in an UPDATE/DELETE, or scalar full select (wherever
used), READ UNCOMMITTED allows:

Any row that is read during the unit of work to be changed by other application
processes.

Any row that was changed by another application process to be read even
if the change has not been committed by the application process.

For other operations, the rules that apply to READ COMMITTED also
apply to READ UNCOMMITTED.